Grassroots: Newsletter of the Grassland Society of Southern Africa ▪ May 2008 ▪ Vol 8 ▪ No.2
Increase of woody plants in
savannah ecosystems
Photo: Peter Scogings
Vhalinavho P. Khavhagali and William J. Bond
University of Cape Town
Email: [email protected]
A
known as “woody plant encroachment” (Trollope 1980). Woody plant
encroachment has occurred in
many parts of the world, including
Africa (Sankaran et al. 2005).
Conversion of savanna woodlands to forest/thicket stands will be
referred to as forest colonization.
Forest colonization is a process
whereby forest/thicket species colonize savannas to form a closed
woody stand. At times, grasslands
and savannas are replaced by
scrub thicket and eventually closed
forest (Bond et al. 2005). Forests
are defined as ecosystems with
large trees and overlapping tree
layers. Forests and other woody
formations differ from savannas in
lacking a continuous grass cover
(Bond et al. 2005). Invasion of forest species in savannas causes a
complete replacement of savanna
biome to a forest/thicket formations,
bringing about a biome shift. Trapnell (1959) and Archer et al. (1988)
reported the natural succession,
invasion of forest species in sa-
lthough savannas cover a
large part of the world’s
land surface, there is still
limited understanding about what
determines the structure and distribution of savannas. Savannas are
broadly defined as tropical seasonal
ecosystems with a continuous grass
layer, mixed with forbs and sedges
with a variable cover of trees and
shrubs. Savannas occur in seasonal climates with a distinct dry
and wet season, and they are important socio-economically in tropical regions (Scholes and Archer
1997).
An increase in woody plant
density has been reported as a
problem in grassland and savanna
ecosystems, because increased
woody cover can result in decreased herbaceous production and
diversity. Trees, shrubs and thicket
species invade open grasslands
through a process well known as
bush encroachment, and thicken up
in already wooded areas to form
woodlands through a process
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Grassroots: Newsletter of the Grassland Society of Southern Africa ▪ May 2008 ▪ Vol 8 ▪ No.2
of forest/savanna boundaries have
long been debated with some arguing strongly for soil and climate limitations, others for fire, and rarely,
an interaction between fire and site
conditions in determining forest distribution (Bond et al. (2003a) for
South Africa). Bond et al. (2003a)
have argued that most of the higher
rainfall eastern grasslands and savannas of South Africa have the
climate potential to support forests.
They suggest that most grassy biomes with >750 mm mean annual
precipitation (MAP) in this region
would switch to forest in the long
absence of fire.
It has been implied that forest
seedling recruitment takes place
beneath canopies of savanna trees
because they increase resource
availability beneath their canopies
(Belsky et al. 1989). For example,
Acacia tortilis and Adansonia digitata have been shown to increase
herbaceous productivity, lower soil
temperatures and increase soil fertility beneath their canopies (Belsky
et al. 1989). Grasses influence
woody plant recruitment indirectly
by promoting a distinct fire regime
with very frequent fires. However
they also have direct effects on
woody plants by competition for
resources, especially in the establishment phase when saplings are
shaded by grasses and roots have
to compete with grass roots.
Several researchers reported
that recruitment of forest species
into savannas is limited by soil nutrients (Kellman 1979), frequent fires
(Bond et al. 2005), and drought or
seasonal water logging (Knoop and
Walker 1985). Establishment of for-
vanna environment resulting in the
formation of forest/thicket stands.
Replacement of savannas by forests is a phenomenon apparently
restricted to mesic areas (>650 mm
rainfall) (Swaine et al. 1992, Bond
et al. 2003a) and seems to be occurring in many such areas in South
Africa (Hoffmann and O’Connor
1999, Bond et al. 2003b) and Australia (Bowman et al. 2001) over the
last half century.
The increase in forest coverage
and biome shift from savanna to
forest ecosystems varies remarkably. It is much more difficult and
costly to reverse the process of forest invasion than to control changes
in abundance of savanna trees or
shrubs. An ecosystem switch from
savanna to forest brings about
changes in species composition,
with grasses shading out, an increase in fire intolerant species and
a total biome shift. Several studies
have proposed a variety of contributing factors, including climate
change, increase in atmospheric
CO2 concentration, fire regimes,
grazing by livestock and wild herbivores, canopy cover, and soil resources as factors influencing
woody plant encroachment (Knoop
and Walker 1985, Bond and van
Wilgen 1996, Higgins et al. 2000,
Bond et al. 2003a, Ludwig et al.
2004, Bond et al. 2005, Sankaran et
al. 2005, Govender et al. 2006).
However, it is not well known
whether these same factors that
influence woody encroachment are
responsible for forest colonization.
Forest and thicket patches are
common, if small in extent, in most
savanna landscapes. Determinants
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Grassroots: Newsletter of the Grassland Society of Southern Africa ▪ May 2008 ▪ Vol 8 ▪ No.2
Frequent increase of woody plants
in grasslands and savanna, and
forest invasion/colonization is taking
place in high rainfall areas in South
Africa. Open grasslands are transformed to open savannas which
thicken up to savanna woodlands
and/or switch from woodlands to
forest/thicket through a process
called forest invasion. Not only an
increase in woody plants, but also a
change in species composition and
a reduction in grasses so that frequent fires and herbivory can no
longer be supported, all result in the
formation of forest clumps. This is a
serious conservation and rangeland
problem with the biome switch resulting in reducing conservation
values of savanna parks and livestock potential.
est species in savannas may also
be limited by high light intensity and
high temperatures characteristic of
the savanna environment
(Hoffmann 2000). As a result, the
establishment and growth of forest
species may be low in open savannas, and may be facilitated by the
presence of adult savannas trees
(Kellman 1979). Increase in woody
encroachment and forest invasion is
attributed to fire exclusion (Swaine
et al. 1992, Bond et al. 2005), fireherbivory interactions (Eckhardt et
al. 2000), facilitation by large savanna trees (Kellman 1979, Belsky
et al.1989, Ludwig et al. 2004), atmospheric CO2 and climate change
(Bond et al. 2003b, Sankaran et al.
2005).
Savanna trees ameliorate soil
moisture deficits and reduce nutrient stress for establishing seedlings
by increasing soil fertility under their
canopies (Belsky et al. 1989,
Ludwig et al. 2004) resulting in the
formation of “fertile islands”. Bond
et al. (2003a) and Bond et al.
(2005) suggested that fire is the
main factor maintaining mesic savannas because the climate can
potentially support closed forests.
Fire suppression can trigger rapid
forest invasion or increase tree
cover by favoring woody seedling
establishment or allowing existing
saplings to escape the flame zone
and grow into adult trees (Higgins et
al. 2000, Bond et al. 2005). High
rainfall indirectly restricts forest invasion because it enables grass
fuel to accumulate to support frequent fires (Higgins et al. 2000) that
burn down tree seedlings and coppice growth.
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